Stupid M3 questions which I'm sure have been answered before...

[geremy]

So I am just about to start building an M3. I have several questions and was hoping to clear them up here. If these questions have already been answered feel free to point me to another site/thread. I think M3, M^3 and MMM are too short search terms but I got nada in thread titles.

My plan is to build an M3 using the AD8610s, with adjustable gain. I would like to use switch-controlled digital potentiometers for both the gain and the volume, but I haven't finished designing that circuit yet (and I also haven't found a readout LCD w/ microcontroller mini board suitable yet) so in the meantime I am going with an RK27 for volume and a gain of 8. I only have low impedance headphones. I was thinking to go a bit higher than 24V, but I am building the amp before the power supply (I have plenty of bench supplies to use in the meantime).

Summary:
AD8610s
Hardwired gain of 8 (adjustable later, no bass boost).
Maybe higher than 24V (will test with benchtop supplies)

My questions:
1) Should I go with a lower R1 value? I have ordered 680 KOhm to test with.
2) Since I am baselining a gain of 8, I was thinking about going with 15 pF C1L and C1Rs, but in the end I will (hopefully) have adjustable gain, so should I just go with 22 pF right off the bat?
3) It makes more sense to me to use C3 in conjunction with R8+ and R8-. I can't really see any benefit to adjusting the value of the two resistors. Am I missing something?
4) Some of the transistors for Q2, Q5+ and Q5- have been obsoleted. I am torn between using (mouser) 864-2n3904g/864-2n3906g combo or an 863-2n4401G/863-2n4403G combo. Is there any difference? Am I correct in that the main parameter to pay attention to is Hfe?

That's basically it. My final plans involve a relay switched input selector and digital volume/gain with LCD readout. If anyone knows where to find either of these three circuits (relay switched inputs, digital volume control, and LCD mini-board with on-board controller) ready-made, I would be appreciative. I really like the twisted pear audio 'darwin source selector', but it is out of stock.

[JBLoudG20]

I really don't see why you'd want to have a digitally controlled gain adjustment. Maybe a switch with 2 or even 3, but why have more than that?

[Nebby]

I think he meant digital volume control

[Mister X]

Check out Dantimax - Remote control audio kits for an input selector + digital volume control.

1) Changes to R1 have very insignificant impact on the performance of the M3.
2) Yes.
3) I do not see any reason to change those either.
4) Maybe your parts list is what is obsolete? Because there are a myriad of 2N3904/2N3906s in stock at Mouser.

What's a supply voltage higher then 24 going to do for you except add more heat?

[Beefy]

A gain of 8 is FAR too high for low impedance phones. With my 32ohm AD900 at a gain of 2, I get less than 1/6 of a turn on the volume knob before it is too loud.

For everything else, stick with AMB's values. He is all knowing.

[Fungi]

I agree with gain@2, unless you know you listen to louder volumes, will never use anything sensitive, will never use anything other than low output sources, or need higher gain for stability with certain opamps.

[geremy]

Thanks for the tip on the Dantimax. Their vol. 2 boards may just be the ticket.

MisterX: sorry to bother again, but when you say 'yes' to question 2, you are agreeing that I should just put in the 22pf for C1L and C1R right off the bat?

Gain of 8 is way too high? I wasn't expecting that! Especially since the default is 11. In rockhopper's page he says "Gain of 11, perfect for a wide variety of headphones, high and low impedance". I figured I was being conservative by starting with 8! Primary phones with this will be Grados, so I guess I will go with a lower gain initially.

Thanks for all the the help so far. I'm sure I will have plenty more questions later.

Oh, anyone have any thoughts about going higher than 24V with the 8610s? Is it worthwhile?

[n_maher]

I would not suggest going higher than 24V unless you have headphones that you know are going to need an enormous amount of voltage swing. 24V isn't that far away from the max the 8610 is rated for too (26V) and it's better to have a little safety margin.

[Beefy]

I would not suggest going higher than 24V unless you have headphones that you know are going to need an enormous amount of voltage swing. 24V isn't that far away from the max the 8610 is rated for too (26V) and it's better to have a little safety margin.

FWIW, my MMM is running just under 27V and AD8610. By the time you get voltage drop across D1 and various other losses before the OPAMPs, you drop 1.5-3.0V according to AMB; in my particular case, the OPAMP is only seeing 23.5V.

[geremy]

Also, Regarding Q2, Q5+ and Q5-, the BOM on the M^3 front page at the AMB site is what lists the obsolete parts. There are other 3904/3906 parts but not from the same vendor. It probably doesn't matter as the specifications are not all that different.

[Pars]

2n3904/3906 are among the most common and generic transistors in existence. Don't worry about it.

[n_maher]

Also, Regarding Q2, Q5+ and Q5-, the BOM on the M^3 front page at the AMB site is what lists the obsolete parts. There are other 3904/3906 parts but not from the same vendor. It probably doesn't matter as the specifications are not all that different.

Yeah the guy who put that together is a jackass.

Or it could be that the BOM is 4 years old and references parts that were obsoleted by ROHS compliance.

[geremy]

I'm certainly not criticizing. And I understand that the RoHS compliance that obsoleted those items. I couldn't find the fairchild 390x series, even in G. I appreciate the BOM you put together and have referred to it extensively.

[n_maher]

I'm certainly not criticizing. And I understand that the RoHS compliance that obsoleted those items. I couldn't find the fairchild 390x series, even in G. I appreciate the BOM you put together and have referred to it extensively.

I didn't take it as criticism, I was just joking around. But a simple search of Mouser using "2n3904" yielded many results including parts man'f by Fairchild.

[geremy]

Oh ok. I saw the other 390x parts but they all had Hfes of 40, whereas the part from the BOM had an Hfe of 100, which is why I picked the ON semi parts. I was asking if that is really the only param. I should really care about.

[geremy]

Another silly question, but not M^3 specific: does solder compound really matter?

[Beefy]

Yes, it does. You just try building an amp without it.

[geremy]

I mean the type of solder compound. I have lots of stuff around, just wondering which types are recommended for audio equipment.

[n_maher]

Quick question: have you ever built anything before? I'm not sure I'd recommend the M3 you're proposing as a first (or even second) project.

[geremy]

Yes. Nothing audio related though. I'm an electrical engineer. I have plenty of standard eutectic solder, but I'm wondering if I the Cardas solder (or equivalent) would represent a significant improvement.